A thermally foamable microsphere is obtained by microcapsulating a volatile foaming agent with a polymer and also called a thermally expandable microcapsule. The thermally foamable microsphere can be generally produced by a process in which a polymerizable mixture containing at least one polymerizable monomer and a volatile foaming agent is suspension-polymerized in an aqueous dispersion medium. An outer shell (shell) is formed by a polymer formed as a polymerization reaction progresses, thereby obtaining the thermally foamable microsphere having a structure that the foaming agent is encapsulated in the outer shell so as to be wrapped in the outer shell.
As the polymer forming the outer shell, is generally used a thermoplastic resin having good gas barrier properties. The polymer forming the outer shell is softened by heating. As the foaming agent, is generally used a low-boiling compound such as a hydrocarbon which becomes a gaseous state at a temperature lower than the softening point of the polymer forming the outer shell. When the thermally foamable microsphere is heated, the foaming agent vaporizes, and the expanding force thereof acts on the outer shell, and also the elastic modulus of the polymer forming the outer shell rapidly decreases. Therefore, rapid expansion occurs bordering on a certain temperature. This temperature is referred to as a foaming start temperature. When the thermally foamable microsphere is heated to a temperature not lower than the foaming start temperature, the microsphere itself expands to form a foamed particle (closed-cell foamed particle).
The thermally foamable microsphere is used in a wide variety of fields as a designing ability-imparting agent, a functionality-imparting agent, a weight-lightening agent and the like making good use of its properties of forming a foamed particle. Specifically, the thermally foamable microsphere is added for use to, for example, polymeric materials such as synthetic resins (thermoplastic resins and thermosetting resins) and rubbers, paints, inks and the like. When high performance comes to be required of the respective application fields, the performance level required of the thermally foamable microsphere is also raised. As an example of the performance required of the thermally foamable microsphere, may be mentioned improvement in processing characteristics.
For example, in foam molding using the thermally foamable microsphere, the thermally foamable microsphere is incorporated into a polymeric material such as a synthetic resin or rubber to form a composition by kneading or calendering, and the composition is then extruded or injected to foam the thermally foamable microsphere in the process thereof.
By this foam molding, a molding or sheet, to which weight lightening has been made, or designing ability has been imparted, can be obtained. Prior to the foam molding, the thermally foamable microsphere may be incorporated into the polymeric material in some cases to prepare pellets by extrusion under conditions that the thermally foamable microsphere is substantially not foamed. Master batch pellets with the thermally foamable microsphere incorporated into the polymeric material may be prepared in some cases, and the master batch pellets may be diluted with the polymeric material to subject the thus-obtained composition to foam molding.
However, the conventional thermally foamable microspheres are generally narrow in foaming start temperature range and initiate foaming at a relatively low temperature, so that the microspheres are easy to cause premature foaming upon kneading or processing such as palletizing prior to the foam molding. Therefore, the processing temperature must be lowered, and so the kinds of applicable synthetic resins and rubbers have been limited.
The foaming start temperature of the thermally foamable microsphere can be controlled by properties of the polymer forming the outer shell, such as glass transition temperature, molecular weight and softening temperature, and the boiling point of the foaming agent. When a high-boiling foaming agent is used for raising the foaming start temperature, the foaming agent comes to have a high molecular weight, so that an internal pressure upon vaporization becomes low, and it is difficult to raise an expansion ratio. In order to raise the internal pressure upon the vaporization using the high-boiling foaming agent, it is necessary to increase the content of the foaming agent. When the content of the high-boiling foaming agent is increased on the other hand, the thickness of the outer shell must be thinned, so that the vaporized foaming agent escapes through the outer shell to lower the expansion ratio. In other words, it is not that the mere use of the high-boiling foaming agent is better for raising the foaming start temperature.
On the other hand, the thermally foamable microsphere is often heated and foamed (expanded) after it is mixed with a binder resin for a paint or ink, or after coating or printing is conducted with the resultant mixture. However, when the foaming start temperature is too high, the decomposition of the paint or the deterioration of the resultant coating film by heat occurs, or difficulty is encountered on a foaming operation, so that the foaming start temperature cannot be made high. When a low-boiling foaming agent is used, it is easy to initiate foaming at a low temperature and raise the internal pressure upon the vaporization.
Therefore, a low-boiling organic compound has heretofore been used as the foaming agent in the technical field of thermally foamable microspheres. Examples of typical foaming agents include hydrocarbons such as ethane, ethylene, propane, propene, n-butane, isobutane, butene, isobutene, n-pentane, isopentane, neopentane, n-hexane, isohexane, cyclohexane and heptane; chlorofluorocarbons such as CCl3F; and tetraalkylsilanes such as tetramethylsilane (for example, U.S. Pat. No. 6,613,810).
In order to obtain a thermally expandable microcapsule (thermally foamable microsphere) expanding at a relatively low temperature without lowering heat resistance and chemical resistance, there has heretofore been proposed a method in which at least two hydrocarbons different in boiling point from each other are encapsulated (U.S. Pat. No. 5,861,214). However, the resultant thermally expandable microcapsule is such that its processing temperature cannot be made high because two or more of such low-boiling hydrocarbons as described above are used in combination even in this method.
When the thermally foamable microsphere is contained in a composition to be processed at a high temperature, a method of using isooctane (i.e., 2,2,4-trimethylpentane) as a foaming agent has been proposed in order to prevent unintended premature foaming during mixing (for example, US 2003-114546 A1, US 2003-143399 A1, U.S. Pat. No. 6,235,394 and U.S. Pat. No. 6,509,384). However, the boiling point of isooctane is 99.25° C. and is not sufficiently high, so that sufficient heat resistance cannot be imparted to the resulting thermally foamable microsphere. A thermally foamable microsphere making use of isooctane alone or a mixture of isooctane and a hydrocarbon having a boiling point lower than isooctane as a foaming agent is not permitted making the foaming start temperature sufficiently high. In addition, the thermally foamable microsphere making use of isooctane as a foaming agent is yet insufficient in that a shrink phenomenon due to gas escaping is prevented.
In general, thermally foamable microspheres involve a problem of the shrink phenomenon due to gas escaping. When a thermally foamable microsphere is heated, the outer shell first starts to soften, and at the same time the foaming agent encapsulated therein starts to gasify to raise the internal pressure of the microsphere into an expanded state. When the heating is further continued, the microsphere starts to shrink because gas passes and diffuses through the outer shell thinned by the expansion.
The conventional thermally foamable microspheres tend to cause rapid foaming. When the softening of the outer shell rapidly occurs upon foaming, the expanded thermally foamable microsphere shrinks again because the vaporized foaming agent passes and diffuses through the outer shell. This phenomenon is referred to as shrink.
In order to prevent the shrink phenomenon of the thermally expandable microcapsules (thermally foamable microspheres), there has heretofore been proposed a method in which a monomer component at least 70% by weight of a nitrile monomer such as acrylonitrile or methacrylonitrile is used to form an outer shell, and a proportion of a volatile expanding agent (foaming agent) having a branched or cyclic structure is controlled to at least 30% by weight (EP 1 564 276 A1). In the method disclosed in this document, however, low-boiling hydrocarbons such as isobutane, isopentane, neopentane, isohexane, cyclohexane and 2,2-dimethylhexane are used as the volatile expanding agent. The thermally expandable microcapsule with the low-boiling hydrocarbon encapsulated therein expands at a low temperature and is low in foaming start temperature, so that a processing temperature prior to foaming cannot be made high. This thermally expandable microcapsule tends to cause shrink at a high temperature after foaming.
As described above, the conventional thermally foamable microspheres have been difficult to make a processing temperature prior to foam molding sufficiently high. In addition, the conventional thermally foamable microspheres have been marked in the shrink phenomenon upon foam molding or under high-temperature conditions. Furthermore, the conventional thermally foamable microspheres have been difficult to control their expansion ratios to a desired value because foaming rapidly occurs.